make Packet::AddTag a const method

src/common/packet.h

@@ -158,15 +158,27 @@ public:
   template <typename T>
   uint32_t RemoveTrailer (T &trailer);
   /**
+   * \param tag a pointer to the tag to attach to this packet.
+   *
    * Attach a tag to this packet. The tag is fully copied
    * in a packet-specific internal buffer. This operation 
    * is expected to be really fast. The copy constructor of the
    * tag is invoked to copy it into the tag buffer.
    *
-   * \param tag a pointer to the tag to attach to this packet.
+   * Note that adding a tag is a const operation which is pretty 
+   * un-intuitive. The rationale is that the content and behavior of
+   * a packet is _not_ changed when a tag is added to a packet: any
+   * code which was not aware of the new tag is going to work just
+   * the same if the new tag is added. The real reason why adding a
+   * tag was made a const operation is to allow a trace sink which gets
+   * a packet to tag the packet, even if the packet is const (and most
+   * trace sources should use const packets because it would be
+   * totally evil to allow a trace sink to modify the content of a
+   * packet).
+   *
    */
   template <typename T>
-  void AddTag (T const &tag);
+  void AddTag (T const &tag) const;
   /**
    * Remove a tag from this packet. The data stored internally
    * for this tag is copied in the input tag if an instance
@@ -450,7 +462,7 @@ Packet::RemoveTrailer (T &trailer)
 
 
 template <typename T>
-void Packet::AddTag (T const& tag)
+void Packet::AddTag (T const& tag) const
 {
   const Tag *parent;
   // if the following assignment fails, it is because the

src/common/tags.cc

@@ -30,7 +30,7 @@ struct Tags::TagData *Tags::gFree = 0;
 uint32_t Tags::gN_free = 0;
 
 struct Tags::TagData *
-Tags::AllocData (void)
+Tags::AllocData (void) const
 {
   struct Tags::TagData *retval;
   if (gFree != 0) 
@@ -47,7 +47,7 @@ Tags::AllocData (void)
 }
 
 void
-Tags::FreeData (struct TagData *data)
+Tags::FreeData (struct TagData *data) const
 {
   if (gN_free > 1000) 
     {
@@ -61,7 +61,7 @@ Tags::FreeData (struct TagData *data)
 }
 #else
 struct Tags::TagData *
-Tags::AllocData (void)
+Tags::AllocData (void) const
 {
   struct Tags::TagData *retval;
   retval = new struct Tags::TagData ();
@@ -69,7 +69,7 @@ Tags::AllocData (void)
 }
 
 void
-Tags::FreeData (struct TagData *data)
+Tags::FreeData (struct TagData *data) const
 {
   delete data;
 }

src/common/tags.h

@@ -44,7 +44,7 @@ public:
   inline ~Tags ();
 
   template <typename T>
-  void Add (T const&tag);
+  void Add (T const&tag) const;
 
   template <typename T>
   bool Remove (T &tag);
@@ -71,8 +71,8 @@ private:
   };
 
   bool Remove (uint32_t id);
-  struct Tags::TagData *AllocData (void);
-  void FreeData (struct TagData *data);
+  struct Tags::TagData *AllocData (void) const;
+  void FreeData (struct TagData *data) const;
 
   static struct Tags::TagData *gFree;
   static uint32_t gN_free;
@@ -96,7 +96,7 @@ namespace ns3 {
 
 template <typename T>
 void 
-Tags::Add (T const&tag)
+Tags::Add (T const&tag) const
 {
   const Tag *parent;
   // if the following assignment fails, it is because the
@@ -116,7 +116,7 @@ Tags::Add (T const&tag)
   void *buf = &newStart->m_data;
   new (buf) T (tag);
   newStart->m_next = m_next;
-  m_next = newStart;
+  const_cast<Tags *> (this)->m_next = newStart;
 }
 
 template <typename T>